1. Field of the Invention
This invention relates to a method and apparatus for aligning and positioning a workpiece, such as a lens, so that an axis of the workpiece is precisely aligned with a fixture and a surface of the workpiece is precisely positioned a constant distance from a reference point on the fixture. The invention has particular application in the field of manufacturing contact lenses.
2. Description of Prior Art
In the manufacture of lenses, the degree of accuracy of alignment of a partially finished lens to its fixture, e.g., a mandrel, determines the limit of concentricity possible through subsequent machining operations. A high degree of concentricity is advantageous in the manufacture of lenses since greater concentricity results in less prism. The accuracy of positioning the apex of the lens to a reference point or shoulder of the fixture determines the accuracy of the manufactured lens thickness. A high degree of consistency of lens thickness is advantageous for the manufacture of lenses.
An example of the use of this invention is in the manufacture of contact lenses. However, the invention is not so limited and the principles of the invention may be applied to the manufacture of other types of lenses or objects other than lenses. For purposes of example only, the process of manufacturing contact lenses will be described.
Contact lenses are typically manufactured by way of a complex multi-step operation whereby the fabricated lens goes through many precision operations. As shown in FIG. 1a, generally the first stage of manufacture forms a polished lens surface 2 on a "blank" 1 of plastic. The second stage (FIG. 16) of manufacture transfers this optical lens surface to a block 3 whereby the polished surface is precisely fixed to a block 3 with a suitable material such as wax or cement 5 so that a second surface of the lens can be machined. This stage is commonly referred to as blocking a lens. The third stage (FIG. 1c) forms a polished optical surface 7 of a fixed diameter on the second surface of the lens 8 thereby forming a contact lens. The fourth stage (FIG. 1d) involves removing the finished lens and polishing the edges 9 of the lens 10 in a known manner.
In order to minimize prism and maximize concentricity of the manufactured lens, and to precisely control the thickness of the manufactured lens, it is important to precisely align an axis 6 of the lens, such as the optical axis, and to ensure that a portion of the lens, e.g., the apex, is precisely positioned a constant distance d from a reference point 4. However, such precise positioning can be time consuming and difficult to obtain if done manually.
While some attempts have been made to automate this process to some extent, the prior art falls short of achieving the results of the present invention.